Power semiconductor devices, such as power bipolar transistors, power MOSFET, and IGBT, are typically utilized to control high voltages and large currents involved in switching such devices as power sources, solenoids, lamps, motor-controlling inverters, and DC-motor switches. These power semiconductor devices have a "safe operation area" (SOA) that corresponds to the magnitude and conduction time of output currents. If a current exceeding the SOA flows for an extended period of time, the power semiconductor devices overheat and thermally breakdown.
In order to prevent such overcurrent conditions, the power semiconductor devices incorporate a protection device to monitor the output current and temperature of the power semiconductor devices. The protection device limits or interrupts the current flow in the case of an overcurrent or overheating condition.
FIG. 4 shows a circuit diagram of an integrated circuit incorporating a conventional overcurrent-protection circuit. An n-channel power MOSFET (1), used as a power semiconductor device, is connected with a current-mirror element (2) acting as a current sensor. The drains of both the power MOSFET (1) and the current-mirror element (2) are connected in parallel to a common, first main-current terminal. The gates of both elements are connected in parallel to a common control terminal (5).
The source of the power MOSFET (1) is connected to a second main-current terminal (4). The source of the current-mirror element (2) is connected to a shunt-current terminal (6).
The first main-current terminal (3) is connected to the higher-potential side of a power source (8) via load (7). The second main-current terminal (4) is connected to the lower-potential side of the power source (8). The power source (8) supplies the main current, I, to the power MOSFET (1) via the load (7). A control signal sent from a drive circuit (9) to the control terminal (5) controls the main current.
The overcurrent-detection element (11), shown in FIG. 4, consists of a detection resistor (12) connected between the second main-current terminal (4) and the shunt-current terminal (6), a constant-voltage device (13), and a comparator (14) connected between the lower-potential side of the constant-voltage device and the shunt-current terminal. A shunt current, i, shunted from the main current at a predetermined ratio by the current-mirror element (2), flows across the detection resistor (12). The constant-voltage device (13), connected between the second main-current terminal (4) and the comparator (14), generates a predetermined, threshold voltage, E.sub.s. The comparator (14), connected between the positive side of the constant-voltage device (13) and the shunt-current terminal (6), compares the potential difference across the resistor (12) with the threshold voltage, E.sub.s. The output of the comparator (14) is transmitted to the control terminal (5) via a control circuit (10) and the drive circuit (9).
In a circuit, such as one shown in FIG. 4, combining an overcurrent-detection circuit with a power MOSFET, the amount of the main current, I, can be ascertained by measuring the potential difference, E, across the detection resistor (12). Measurement of the main current is possible because the ratio of the shunt current, i, flowing into the current-mirror element (2), relative to the main current, is predetermined.
FIG. 5 is a characteristic graph showing the relationship between the shunt current, i, and the potential drop across the detection resistor, for the circuit shown in FIG. 4. It is understood that, for the purposes of the circuit shown in FIG. 4, the ratio i/I is equal to 1/10,000, and the resistance value of the detection resistor (12) is 500 .OMEGA..
In the figure, assuming the upper limit of the main-current value in the SOA to be 2A, the main current at the SOA can be determined to be 2A by finding the point, P1, on the curve which corresponds with the shunt-current value of 200 .mu.A and the inter-terminal potential difference of 0.1 V between the shunt-current terminal and the second main-current terminal. Consequently, if the threshold voltage E.sub.s of the constant-voltage device (13) has been set to 0.1 V, one can identify the main current as having reached an overcurrent state when the comparator determines that the inter-terminal potential difference, E, exceeds the threshold voltage of 0.1 V.
As a result of incorporating the overcurrent-detection element, overheating and breakdown failure of a power MOSFET can be prevented by appropriately adjusting the main-current flow to the output signal of the comparator. An overcurrent-detection signal, V.sub.o, is transmitted to the control circuit (10) whenever the comparator satisfies the condition of E-E.sub.2 &gt;0. Based on the overcurrent-detection signal, V.sub.o, the control circuit (10) controls via the drive circuit (9) the voltage at the control terminal (5), thereby performing a protective operation of either limiting or interrupting the main current, I.
If the entire circuit shown in FIG. 4 could be integrated on a single chip with the use of conventional technology, there would be a great economic advantage. However, even if such integration is possible, the inherent variance or production tolerance as affecting the accuracy of the comparator, in responding to the offset voltage applied to ascertain whether an overcurrent condition exists, will greatly affect the overcurrent-detection performance of the overcurrent-detection circuit.
The variance or tolerance of a comparator made in a conventional manufacturing process suitable for production of power IC, in responding to the offset voltage, typically reaches .+-.10 mV. When this figure is converted to a variance in terms of detection accuracy of the detection or shunt current, i, in FIGS. 4 and 5, the variance is .+-.20.mu.A. In other words, the variance or tolerance results in a determination error of .+-.10%.
If an attempt is made to use a comparator with less variance in responding to the offset voltage, such comparator will not be compatible with the tolerance inherent in the conventional power-IC production process. Manufacturing such a comparator to closer tolerances, for example, in a separate production process involving separate chips for the power IC and comparator, will be an economic burden.
In attempting to find a way to reduce the effect of the variance of a comparator in responding to the offset voltage so as to permit satisfactory detection accuracy with a single chip, one method which might be considered might involve raising the resistance value of the detection resistor (12) and increasing the inter-terminal voltage drop due to the shunt current, i. However, increasing the resistance will affect the shunt ratio of the shunt current, i, relative to the main current, I, thereby preventing improvement in accuracy of overcurrent determination.